In an OFDM receiver, channel smoothing is performed on the estimated channel in order to reduce the effects of noise on the estimated channel, thereby improving the system packet error performance. FIG. 1 shows a single stream OFDM transmitter 102 accepting an input stream s1 104 to a baseband encoder 106 which encoded stream is provided to an inverse fast Fourier transform (IFFT) 108 to produce a plurality of baseband subcarriers such as 1 through 1024 or 1 through 512, and the subcarriers are modulated to a carrier frequency for coupling to an antenna 112 as transmitted signal X. The transmitted signal X is coupled through a channel with a frequency dependent characteristic H to receive antenna 132 of receiver 130 to form received signal Y=HX. The receiver 130 receives signal Y, which is baseband converted using RF Front End 133 and applied to FFT 134 to channel compensator 138 and to decoder 140 which generates the received stream S1′. Channel estimator 136 estimates the channel characteristic H during a long preamble interval, and the channel characteristic H is applied to channel compensator 138.
FIG. 2 shows a Multiple Input Multiple Output (MIMO) receiver 240 operative on two transmit streams s1 and s2 204 encoded 206 and provided to first stream IFFT 208 which generates baseband subcarriers, which are provided to RF modulator and amplifier 210 and coupled as X1 to antenna 216. Second stream IFFT 212 and RF modulator and amplifier 214 similarly generate subcarriers which are upconverted and coupled to antenna 218 as X2. Receiver 240 has three antennas 242, 244, 246, which couple to receivers 248, 250, 252 and to output decoder 254 which forms decoded streams s1′ and s2′. Each receiver 248, 250, 252 performs the receive functions as described for FIG. 1, however the channel estimation function 249, 251, 253 for each receiver uses the long preamble part of the packet to characterize the channel from each transmit antenna 216, 218 to each receive antenna 242, 244, 246. For example, receiver 248 must characterize and compensate the channel h11 from 216 to 242 as well as channel h12 from 218 to 242. Each channel characteristic h11 and h22 is a linear array containing real and imaginary components for each subcarrier, typically 1 through 1024. The channel estimator 249 therefore contains h11 and h12, estimator 251 contains h21 and h22, and channel estimator 253 contains h31 and h32. The 2×3 MIMO case of FIG. 2 shows the case where the number of remote transmitters Nt=2 and the number of local antennas and receivers Nr=3. For a MIMO receiver where the number of remote transmitters is Nt and the number of local antennas and receivers is Nr, the Nt*Nr channels have a frequency response which may be smoothed over a range of subcarrier frequencies using a finite impulse response (FIR) filter for I and Q channels. Such a channel smoothing filter would require a total of 2*Nt*Nr filters. For a 13 tap FIR filter, each tap would have an associated multiplier, so such an implementation would require 13 complex multipliers for each filter, or 26*Nt*Nr multipliers total at each receiver station.
It is desired to reduce the complexity of required filtering for a MIMO OFDM channel equalization system. It is also desired to form a channel smoothing filter which does not require multipliers. It is also desired to provide a channel smoothing filter which operates by iterating over all of the FFT receive channels.